Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Capacitors, why even bother?

  1. Dec 20, 2012 #1
    I just finished my last physics class (e&m) and I was pretty intrigued by capacitors and their ability to store charge. I decided to take an electronics class next quarter so I bought a few things including breadboard, capacitor, resistor, diodes, motors, and so on.
    Now...I have a 50V 1000uF capacitor. I filled(?) it with 24V (the motor I have is 3-24V range) and connected it to my motor, the motor spins for like 1/10th of a second. Then the capacitor goes down to like ~1V.
    I did a little research on capacitor tanks, and even then, from what people write, capacitors don't last longer then 10 seconds. What gives? Why even use capacitors to store charge. Only useful application I can see is to implement them into circuits with a transformer to make the current a little less choppy.

    I had my hopes set out on harnessing solar energy using photovoltaics and storing the energy in a capacitor, but now that seems like it isn't gonna work.
    Are rechargeable batteries a better option to store charge?

    If someone can give me a reason on why capacitors are useful for storing charge that would be appreciated, along with answering the above.


    Thank you for your help (sorry for the wall of text).
     
  2. jcsd
  3. Dec 20, 2012 #2
    Batteries are used for long term storage of electrical energy as you know.

    I'm a near noob, but caps are used on small time scales, they are always getting charged/discharged, and they can be used to do a pile of things, but they are mostly always charging/discharging when taking part in whatever

    1 basic thing is, to use a water analogy, say you where using a hose but the water pressure kept changing at random, but you wanted a smooth steady flow, a capacitor acts sort of like a little tank that the random hose would fill, but at the bottom had a steady steam coming out of it's tap.

    So for a voltage that jumps up and down a bit, a capacitor is used to smooth it to a steadier level
     
    Last edited: Dec 20, 2012
  4. Dec 20, 2012 #3
    Right. A camera flash circuit uses a capacitor to release a high amount of energy in a short amount of time. So that means capacitors are only good for short bursts of energy?
     
  5. Dec 20, 2012 #4

    nsaspook

    User Avatar
    Science Advisor

    No, it means that capacitors obey the rules of electrical charge movement. Batteries and capacitors can complement each other in a energy storage system. Batteries usually have a limited number of charge/discharge cycles before they start to fail and most batteries are not very efficient in storing energy when almost full. By adding a large capacitor bank that's discharged first instead of the batteries you can reduce the wear on the batteries and because the storage efficiency of a capacitor is usually the same at any point it can increase system efficiency if the load consists of a many short bursts like in a EV stuck in stop and go traffic with a regeneration system that captures the braking energy.
     
  6. Dec 20, 2012 #5
    cpatel23,

    Rechargable batteries and capacitors do not store charge, they both store energy. Knowing that, what is your question?

    Ratch
     
  7. Dec 20, 2012 #6
    In terms of storing back up energy (for blackouts/being self sufficient), would it be better to use a battery or capacitor. From what I've learned, batteries would be more efficient even though they have their limitations.
    This may seem stupid, but is it possible to store energy in a generator and then withdraw it when needed? Or is that the same as a battery?

    So from what I understand, if I have a breadboard whose primary function is to rotate a motor, it would be more efficient to have a capacitor give the initial push, and then use the battery to keep it going. Is this true?




    thanks for your responses.
     
  8. Dec 20, 2012 #7
    cpatel23,

    Several factors need to be considered, physicsl size, voltage, cost, and energy capacity.

    Efficiency is not a consideration. Other factors listed above are more important,

    No, batteries can store energy chemically, capacitors can store energy in a electrostatic field, but a generator converts mechanical energy into electrical energy. So unless you can spin a flywheel connected to a generator, you cannot store energy in a generator.

    Ratch
     
  9. Dec 20, 2012 #8
    Storing charges is only one of the many use of capacitors. Powering a motor or any circuit is not exactly an important use of capacitor. There are so many things involve capacitance that battery cannot do.

    1) Cap don't pass DC current and voltage, so it is use to block DC from reaching from one side to the other. But at the same time, it can pass AC signals, so it can couple the AC signal from one stage to another.

    2) In electronics, we use capacitor and resistors to form poles and zeros to create low pass and high pass networks.

    3) Read up transmission line theory, it's all about L and C.

    4) filter out ripple of the un regulated voltage source.

    These are just few comes to mind, there are a lot more use that basic EM....or even advance Electrodynamics don't get into. You really need to take a class in basic electronics.
     
    Last edited: Dec 20, 2012
  10. Dec 20, 2012 #9
    There are many, many other applications where capacitors are uniquely qualified. For example capacitors can be made to charge and discharge *very* rapidly, much more rapidly than any battery. Capacitors used tune radio antennas are charged and discharged *billions* of times per second continuously.
     
  11. Dec 20, 2012 #10
    I was thinking of a small scale setup (until I really get the hang of things). My initial idea was to by a dozen or so 1.2V/9V rechargeable batteries. Putting them in series accordingly to achieve a needed voltage, recharging them using solar panels. As for cost, I understand they could get a little pricey but in my opinion, it's worth the experience/knowledge.

    When you said energy capacity, do you mean Watts or amp hour?
    Since Watts have been brought up, lets say I have something that requires a minimum amount of Watts, but I only supply 70% of the minimum. Would my appliance work?
     
    Last edited: Dec 20, 2012
  12. Dec 20, 2012 #11
    What do you mean capacitors do not pass DC Volts/Amps? My multimeter gives me a legitimate reading when I measure DC voltage in a capacitor. And how can you use a capacitor to block DC current from going backwards, I always thought that is what diodes are for.
     
  13. Dec 20, 2012 #12
    I am kind of taken back by the example and the question!!! I don't mean to be sarcastic.....I don't mean to. But this is like saying a TV can light up a dark room. But it is not that bright and it flicker, so what is the use of a TV!!!! I am just kind of lost of words. Using in motor is only one of the many many.....many use of a cap.

    Yes, capacitor is that useful and that important.
     
    Last edited: Dec 20, 2012
  14. Dec 20, 2012 #13
    You put 5V on one side, and if you connect a very high value resistor from the other side to ground, you won't measure any voltage on the side with the resistor to ground. The cap block the DC voltage. But if you put a high frequency signal on one side of the cap, you can see the signal on the other side, it passes the AC signal.
     
  15. Dec 20, 2012 #14
    Lol, please forgive me, only education I have on electronics is from my physics classes. Taking an introduction to electronics this upcoming quarter. Hopefully it can clear things up.
    Any good electronics books in mind that I can read until then?


    Also, when I use a DC voltage regulator to regulate the voltage from my transformer (25V down to 15V) it gets really hot. Is this normal?
     
  16. Dec 20, 2012 #15
    Someone need to give you a name of an introduction book, I don't have one right off my head. You are dropping to much voltage ( from 25 to 15V) across the regulator. Remember W=IV. if you drop 10V across the regulator, and if you even draw 100mA, it is 1W power dissipating with the regulator. If you don't have a heat sink, it can get hot. Some of the LM7805 get hot even when you are not drawing current.

    I know a very good book that is a step above AC/DC circuits. It's the Electronic Principle by Malvino. I just bought a copy for under $10USD shipped. Go on Amazon and look for a used one.
     
  17. Dec 21, 2012 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    cpatel -Your interest and enthusiasm are refreshing. continue experimenting.


    Tinkering with capacitors can give you that intuitive "feel" for the meanings of

    charge, , Q, Coulombs, a coulomb being around 6E18 charge carriers(usually electrons in our business)

    current, I, Amperes, amount of charge moving past a point per unit time, one ampere being one coulomb per second

    voltage , Joules per coulomb, energy per unit of charge, one volt being one joule per coulomb

    energy , joules or watt seconds, amount of work that can be done, one joule being one newton-meter

    capacitance, Farads,, C a measure of the ease with which a capacitor stores charge. Units of capacitance are curiously same as length because it's area (of plates) divided by distance separating them, more later

    Capacitors do store charge, in the amount Q = C X V
    in other words one coulomb will charge one farad to one volt

    That leads to concept of DC current into a capacitor-
    one ampere into one farad will raise its voltage one volt per second
    but it's not practical for that to go on for very long, voltage becoes too high.


    Capacitors, by storing charge, also store the energy which pushed the charge into the capacitor, in amount 1/2 C X V^2
    in other words one farad charged to one volt holds a half-joule
    charged to two volts it holds two joules
    charged to three volts it holds four and a half joules, etc
    it's analogous to compressing a spring

    The capacitor stores the energy in the dielectric separating the plates. There is an attraction between the plates because of their charge, so pulling the plates apart would add energy to the capacitor and this would show up as increased voltage.
    The formula for capacitabce is C= ε X A / D
    A and D being area and distance between plates, and ε is the dielectric property of the material between the plates. Doubling the distance would halve the capacitance. Practical capacitors have small separation between the plates and dielectrics with high value.

    Remain curious my friend, and enjoy electronics.
    www dot discovercircuits dot com has a forum that is popular with hobbyists and students alike, check it out.

    EDIT to your original question

    Capacitors are universally used in converting wall power AC into direct current for electronics.
    AC runs through a rectifier which turns it into a series of positive (or negative) pulses. The capacitor stores charge to be delivered between pulses, thereby providing a fairly constant flow of charge(current) instead of pulses.
     
    Last edited: Dec 21, 2012
  18. Dec 21, 2012 #17

    nsaspook

    User Avatar
    Science Advisor

    With proper design that's true if you plan to recapture energy with a lead-acid battery system as most of the efficiency problems I talked about are mainly due to it's poor charge efficiency above 80% SOC and limited charge/discharge cycles times.

    http://web.mit.edu/first/kart/#concept
    http://web.mit.edu/first/kart/circuit.jpg
     
  19. Dec 21, 2012 #18

    mheslep

    User Avatar
    Gold Member

  20. Dec 21, 2012 #19
    :bugeye: wow. Is there a reason why all the capacitor tanks have low voltage (sub 9V)?

    Are these any good? >>>Heat Sink It says 1W, is that how much it dissipates or how much energy it requires to operate?

    So a rectifier doesn't completely convert AC to DC? It just creates pulses that flow in one direction?

    That's some awesome stuff.

    A question about putties batteries in series with capacitor.
    This would be putting batteries in series with a capacitor to light an LED:
    http://i.imgur.com/7BK9C.jpg

    and this would be destructive
    http://i.imgur.com/S4mWt.jpg

    right?

    and does the order matter? for instance battery, battery capacitor would be the same in terms of output as capacitor, battery, battery?
    Sorry for all the questions, had to many, what seemed to be, close calls yesterday.
     
  21. Dec 21, 2012 #20
    Before you hurt yourself, start here = Introduction to Electronics
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Capacitors, why even bother?
Loading...